15.Having worked all day,most people find themselves(71)wishing(wish) for sleep.(72)Although/Thoughthe exact reasons why people sleep are still being debated,researchers (73)have come(come) up with a number of theories to explain the functions of sleep.First of all,sleep(74)is believed(believe)to give the body time to repair burned-out brain cells and make more of the special chemical that makes(75)itpossible for the brain to think.Another theory holds that sleep enables the body to save energy when we sleep.Because the body temperature is(76)lower(low),less energy is needed to create heat.This method of energy conservation may have helped people survive thousands of years ago(77)whenfood was hard to find.Sleep may also have helped humanity survive by keeping people(78)outof trouble.In prehistoric times,when many large predators(食肉動物) hunted in darkness,the habit of sleeping at night helped prevent people form being an animal's dinner.Scientists also believe that sleep,in addition to being a survival method,is used to improve memory.It allows that brain (79)to forgetforget) or unlearn things that are not necessary,(80)otherwise/orthe mind would become filled with unneeded information.